snapped rear derailleur hanger
Recently I got some new FSA SLK carbon cranks to replace my Ultegra ones. I noticed that the width is slightly different (still can't quite get the front derailleur perfect with them, it rubs ever so slightly when in the small ring/big cog combo).
More of a concern was the way the chain would occasionally "slip" when I shifted down to the small ring. Usually I shift two cogs smaller, then shift the front gear when I do that. Literally the chain wouldn't engage and the cranks just spun uselessly, as if it fell between two cogs at the back. A few pedal strokes and a quick upshift of the rear derailleur and it was fine.
Then today, I'm riding up a hill, nothing too crazy, decide to shift to the small ring -- not under heavy load, I always consciously let up slightly when shifting between rings -- and the rear derailleur hanger snaps completely off sending the whole mess into my spokes. I stopped and disentangled it, the chain is fine, the derailleur is fine, the hanger is sheared in two.
Doing some searching, it sounds like chain suck is the culprit? But the rings are new, the chain is new, cassette is in good shape. This problem has only started since I got the FSA crankset with its narrower width.
Any idea what happened, why, and how to prevent it in the future?